1. Field of the Invention
The present invention relates to an optical pickup system, an optical head, an optical disk apparatus, and an objective lens.
2. Description of Related Art
Conventionally, a compatible optical disk apparatus capable of reproducing different types of optical disks such as compact disks (CDs) and digital versatile disks (DVDs) has been proposed. The CD and DVD (which is hereinafter collectively called the optical disk) both have a transparent substrate whose one side is an information recording surface. The optical disk is composed of a combination of two transparent substrates adhered together with their information recording surfaces facing each other, or a combination of the information recording surface and a transparent protection substrate adhered together with the information recording surface facing the protection substrate.
To reproduce information signals stored in the optical disk, the optical disk apparatus focuses a laser beam from a light source on the information recording surface of the optical disk through the transparent substrate. As detailed later, the wavelength of the laser beam differs between CD and DVD. The optical disk apparatus normally uses an objective lens for focusing the laser beam. In some cases, the thickness of the transparent substrate having the information recording surface differs according to the type of the optical disk or a difference in laser beam wavelength. For example, while the transparent substrate of a CD may be 1.2 mm in thickness, that of a DVD may be 0.6 mm.
For the optical disk apparatus to reproduce optical disks of different types, it is required to focus the laser beam on the information recording surface despite that the thickness of the transparent substrate varies by the type of the optical disk. Besides, a new optical disk apparatus that uses a blue laser of approximately 400 nm wavelength to reproduce information is recently proposed. Therefore, there is a need for the optical disk apparatus to be compatible with the new optical disk in addition to CD and existing DVD.
To meet this need, an optical disk apparatus may have objective lenses for different types of optical disks in a pickup so that the objective lenses are changed in accordance with the type of the optical disk in use. Alternatively, it may have pickups for different types of optical disks so that the pickups are changed in accordance with the type of the optical disk in use. However, in terms of cost and size reduction, it is preferred to use the same objective lens for any type of optical disk.
An example of the objective lens is disclosed in Japanese Unexamined Patent Publication No. 09-145995. This objective lens has a lens surface that is radially sectioned into three or more loop zones. Every other loop zonal lens surfaces and the other every other loop zonal lens surfaces have a different refractive power. The every other loop zonal lens surfaces focus a laser beam on the information recording surface of the optical disk (DVD) having a thin transparent substrate (0.6 mm). The other every other zonal lens surfaces focus the laser beam having the same wavelength, for example, on the information recording surface of the optical disk (CD) having a thick transparent substrate (1.2 mm).
Another example disclosed in Japanese Unexamined Patent Publication No. 2000-81566 (U.S. Pat. No. 6,118,594). It discloses an optical disk apparatus that uses a laser beam having a short wavelength (635 nm or 650 nm) for a DVD with a thinner transparent substrate and uses a laser beam having a long wavelength (780 nm) for a CD with a thicker transparent substrate.
This optical disk apparatus has an objective lens used in common for these laser beams. The objective lens has a diffractive lens structure with a plurality of minute loop zonal steps thickly formed on one side of a refractive lens having a positive refractive power. The diffractive lens structure is designed so as to focus diffracted light of a laser beam having a short wavelength on the information recording surface of a DVD with a thinner transparent substrate, and focus diffracted light of a laser beam having a long wavelength on the information recording surface of a CD with a thicker transparent substrate. Further, it is designed so as to focus the diffracted light having the same diffractive order on the information recording surface. A laser beam with a short wavelength is used for DVD because the recording density of the DVD is higher than that of the CD, thus requiring a small beam spot. As well known, the diameter of an optical spot is proportional to a wavelength and inversely proportional to a numerical aperture (NA).
The above optical disk apparatus allows use of a common objective lens for both DVD and CD. It eliminates the need for replacing components such as an objective lens for each use of DVD and CD. This is effective in reducing costs and simplifying the structure.
Further, Blu-ray disk and High Definition DVD (HD DVD) are appeared as optical recording media in the next of CD and DVD, and optical pickup apparatus that are compatible with the three optical recording media are under development. The optical pickup apparatus is required to transmit the output of a laser light source to a disk of a recording medium at high efficiency. An important point of development to meet this requirement is an antireflection coating that is formed on an optical component such as an objective lens included in the apparatus.
The wavelength of light used in CD is approximately 790 nm, the wavelength of light used in DVD is approximately 655 nm, and the wavelength of light used in Blu-ray disk and HD DVD is approximately 405 nm. Thus, an objective lens placed in the optical pickup apparatus preferably has an antireflection coating that has the optical property that a reflectance is low in the vicinity of these three wavelengths. As an antireflection coating compatible with a plurality of wavelengths an antireflection coating that is compatible with the wavelengths used in DVD and Blu-ray disk is disclosed in Japanese Unexamined Patent Publication No. 2005-38581.
However, since the technique disclosed in Japanese Unexamined Patent Publication No. 09-145995 uses different loop zonal lens surface of the objective lens for DVD and CD, a large area remains ineffective for an incident laser beam, which extremely lowers light use efficiency.
Further, since the technique disclosed in Japanese Unexamined Patent Publication No. 2000-81566 uses the diffracted light by the diffractive lens structure, it is impossible that the diffractive efficiencies for different wavelengths reach 100% at the same time. This diffractive lens is designed so that the diffractive efficiency reaches 100% at an intermediate wavelength between the laser beam with a short wavelength (635 nm or 650 nm) used in DVD and the laser beam with a long wavelength (780 nm) used in CD, thereby making the diffractive efficiency well balanced for the laser beams in use.
Besides, this technique requires minute steps to be formed on the lens surface to create the diffraction lens structure, which is vulnerable to processing error. If the diffractive structure is not formed as designed, it causes a decrease in diffractive efficiently. When the diffractive efficiency decreases or it does not reach 100%, it means incapability of focusing entire incident light on the information recording surface of the transparent substrate in the optical disk, which results in light loss.
Further, Blu-ray format that uses a blue laser having a still shorter wavelength is proposed recently. Backward compatibility is also required for this case. In this case, a wavelength difference is larger than that between DVD and CD, and a difference in the refractive index of a lens is also large. Therefore, in the conventional techniques described above, it is even more difficult to obtain a suitable wavefront aberration in any medium.
The antireflection coating described in Japanese Unexamined Patent Publication No. 2005-38581 defines light transmittance by focusing on two kinds of wavelengths: the wavelength of light used in DVD and the wavelength of light used in Blu-ray disk. In this case, it is possible to form an antireflection coating with low reflectance in the wavelength region of the light used in DVD and Blu-ray disk. However, since the reflectance in the wavelength region of the light used in CD becomes higher than that before formation of the antireflection coating in this example disclosed therein, defect can occur when a CD is used as a recording medium.
For example, when writing information to CD by raising the magnification of a writing speed, use of an antireflection coating that has high reflectance in the wavelength region of the light used in CD is likely to cause a writing error. The magnification of the writing speed of a CD-R drive is increased to as high as 52, and writing at a high magnification is expected.
Further, the technique of Japanese Unexamined Patent Publication No. 2005-38581 expects the use of three or more layers of antireflection coating as well. Since a time to form an antireflection coating in a manufacturing process is proportional to the number of layers of the antireflection coating, the number of antireflection coating is preferably small. When forming a film with low reflectance in two or more kinds of wavelength regions, two layers of antireflection coating called V-coat having V-shaped spectral reflection characteristics can be used if the two kinds of expected wavelength regions are relatively close to each other. However, since the wavelength region of the light used in Blu-ray disk and the wavelength region of the light used in DVD and CD are not close to each other, it is impossible to achieve a purpose with the V-coat.
The antireflection coating is normally placed in the objective lens surface in the optical pickup apparatus. The objective lens is generally made of plastic material fir its high optical performance and low costs. Though the lens made of plastic material is weak against heat, the antireflection coating is formed by vapor deposition or sputtering, and a temperature on the formation surface thereby increases. Since a film formation time is proportional to the number of layers of the antireflection coating, it is preferred that the number of antireflection coating is smaller also in terms of shortening a formation time to reduce effects such as heat deformation on the formation surface.